This invention relates to a method and an apparatus for detecting a tracking error, i.e., a deviation of a light spot projected by an objective lens on a record medium with respect to an information track on the record medium, viewed in a direction perpendicular to an optical axis of the objective lens as well as to the information track.
For instance, there has been proposed an optical disc reproducing apparatus in which an information signal recorded as a series of pits along spiral or concentric information tracks on a disc shaped record medium is reproduced optically by projecting a light spot by means of an objective lens onto the record medium. To reproduce the information signal stably and reliably, it is necessary to decrease radial deviations of the reading light spot on the information track of the disc. To this end, it is first required to derive a tracking error signal representing a magnitude and a direction of the deviation stably and precisely without being affected by disturbances.
FIG. 1 is a perspective view showing an embodiment of a known optical disc player disclosed in U.S. Pat. No. 3,909,608. In FIG. 1, a record medium shaped like a disc is of a transmission type and rotates about a spindle 3 by means of a motor 2. On the disc 1 there are spiral or concentric information tracks 5, each consisting of an array of pits 4. A parallel light beam 7, emitted from a light source 6 along an optical axis Z, is focused by an objective lens 8 onto a point O on the track, thereby forming a very small light spot. The focused light is transmitted through the transparent disc 1 and the diverging light flux is incident upon a region 11 on a light detector comprising two light receiving regions 9 and 10 arranged in a side by side relationship. The light receiving regions 9 and 10 are divided along a line parallel to an axis OX which passes through the point 0 to be read out and is tangential to the information track 5.
Output signals derived from the light receiving regions 9 and 10 are supplied to a differential amplifier 12 to produce a first difference signal which is then supplied to a low pass filter 13. The low pass filter 13 produces a tracking error signal .epsilon. which is supplied to an electrical-mechanical transducer 14 for moving the objective lens 8 in a disc radial direction, i.e., OY direction so as to correct the deviation of the light spot with respect to the information track 5 in the tracking direction.
At the same time, the output signals from the light receiving regions 9 and 10 are summed up by a summing circuit, comprising resistors 15 and the summed up signal is amplified by an operational amplifier 17, having a feedback resistor 16, thereby producing an information signal S(t). In this manner, the information signal is reproduced from the disc shaped record medium 1.
In the above apparatus, when the point O at which the light spot is focused is situated on an intermediate area of successive pits 4 in the same track, no diffraction is present, resulting in the light receiving regions 9 and 10 receiving the medium radiant energy within the region 11. Contrary to this, when the light spot is incident upon a boundary of the pits 4, the light beam is diffracted and radiant energy is then incident upon a hatched region 18 which is wider than the region 11. Therefore, the amplitude of the sum signal S(t) of the outputs from the two light receiving regions 9 and 10 varies while the light spot scans the information track 5. The sum signal S(t) has a substantially rectangular waveform which corresponds to the passage of the light spot through the pits 4 and thus accurately represents the information signal recorded on the disc 1 along the information track 5.
FIG. 2 is a cross section showing another embodiment of a known optical disc reproducing apparatus of a reflection type. In this embodiment an information record medium 19 is made of opaque material. A parallel light flux 7, emitted from a light source (not shown), is transmitted through a half mirror 20 and subsequently focused by an objective lens 8 onto the record medium 19 as a light spot. Light reflected by the record medium 19 is collected by the objective lens 8 and formed into a parallel light beam which is then reflected by the half mirror 20 toward the light receiving regions 9 and 10, which are arranged in a side by side relationship. When the light spot is incident upon an intermediate region between successive pits, the production of a reflected light beam having an intensity which is substantially equal to that of the incident light beam occurs. However, when the light spot is incident upon the boundary of the pit, the light is diffracted outside the objective lens 8, as shown by light rays 21, resulting in a decrease in an amount of the light impinging upon the light receiving regions 9 and 10. Therefore, by processing the output signals S.sub.1 and S.sub.2 from the light receiving regions 9 and 10, by means of a circuit similar to that shown in FIG. 1, it is possible to obtain the tracking error signal and the information signal.
In the aforediscussed apparatuses, the objective lens 8 moves to effect the tracking control and the light receiving regions 9 and 10 are fixed. When no tracking error exists, an optical axis of the objective lens 8 conincides with the bisecting line of the light detectors 9 and 10 and the light flux is incident upon the two light receiving regions in a symmetrical manner. When the objective lens 8 moves in accordance with the detected tracking error signal .epsilon., the symmetrical relationship does not occur. Hence, the tracking error signal may include erroneous components other than the pure tracking error signal, resulting in inaccurate tracking of the disc. The problem may be solved by simultaneously moving the light detector 9 and 10 with the objective lens 8. However, this movable mechanism is complicated in construction, large in size, heavy in weight and it is difficult to incorporate a wide frequency range response into the mechanism. Further, even if such a mechanism for moving the light detector as well as the objective lens is realized, when the record medium 1 is inclined with respect to the optical axis, the light beam incident upon the light detector is shifted, resulting in the same problem. The above problem also occurs in the reflection-type disc reproducing apparatus.
FIG. 3 is a schematic view illustrating how to shift the light beam reflected by a record medium 1 when the record medium is tilted. When the record medium 1 is positioned at a focal point of an objective lens 8, having a focal length f, and inclined by an angle .DELTA..theta., as shown by the broken line 1', a center axis of the light beam reflected by the record medium 1' is deflected from the optical axis of the objective lens 8 by an angle of 2.DELTA..theta. resulting in the reflected light beam shifting by a distance equal to f.multidot.2.DELTA..theta..
The above phenomenon is further explained herebelow.
FIG. 4 is a perspective view showing still another embodiment of a known optical disc reproducing apparatus. An objective lens 8 is held by an actuator 31 and is moved in a Z direction (focusing control) as well as in a radial direction r (tracking control) tangential to a track on a disc shaped record medium 19. A light detector 32 comprises four divided light receiving regions PDa, PDb, PDc and PDd, each being formed by, for example, a PIN photodiode. By suitably processing output signals from the four light receiving regions, it is possible to derive an information signal, a focusing error signal and a tracking error signal.
A polarized light beam emitted from a semiconductor laser light source 35 is collimated by a collimator lens 36 and is reflected by a polarizing prism 33. The reflected light is transmitted through a quarter wavelength plate 30 and is focused by the objective lens 8 onto the record medium 19 as a light spot. The light beam reflected by the record medium 19 is collected by the objective lens 8 and is transmitted through the quarter wavelength plate 30. Since the light beam has been passed through the plate 30 twice, the light beam is transmitted through the polarizing prism 33 and is made incident upon a detection prism 34 having major surfaces set at a critical angle with respect to a center light ray of the beam. The light beam departing from the detection prism 34 is incident upon the light detector 32. The construction and operation of this known apparatus has been fully explained in the specification of U.S. patent application Ser. No. 314,482 filed on Oct. 23, 1981, in which there is further disclosure of how to derive the tracking error signal and focusing error signal. Here, a method of detecting the tracking error will be explained briefly.
FIG. 5 is a block diagram showing a circuit for producing RF information signals and focusing and tracking error signals. The light receiving regions PDa, PDb, PDc and PDd of the light detector 32 are divided so that a direction t, corresponding to a tangential direction of the information track, bisects PDa, PDd and PDb, PDc and a direction r, corresponding to the radial direction of the record medium 19, bisects PDa, PDb and PDc, PDd. Now it is assumed that the four light receiving regions PDa, PDb, PDc and PDd produce outputs a, b, c and d, respectively. A sum S.sub.1 of the outputs a and c is formed by an adder 37 and a sum S.sub.2 of the outputs b and d is derived by an adder 38. Further, a sum S.sub.3 =S.sub.1 +S.sub.2 =a+b+c+d is formed by an adder 39. This sum S.sub.3 represents a reproduced RF information signal. To derive the tracking error signal, a difference S.sub.4 =S.sub.1 -S.sub.2 =(a+c)-(b+d) is formed by a subtractor 40. If there is no tracking error, S.sub.1 is equal to S.sub.2 and therefore, the difference signal S.sub.4 is zero. Contrary to this, if there is a tracking error, the difference signal S.sub.4 is not equal to zero and its phase is advanced or lagged by 90.degree. with respect to the RF signal S.sub.3, in accordance with the direction of the tracking error. Therefore, by detecting the phase of the difference signal S.sub.4 with reference to the RF signal S.sub.3, it is possible to know the direction of deviation due to the tracking error. For this purpose, the RF signal S.sub.3 is supplied to a positive going zero cross detection circuit 41 and a negative going zero cross detection circuit 42 to produce pulses S.sub.5 and S.sub.6, respectively, at zero cross timings. These pulses S.sub.5 and S.sub.6 are used as sampling pulses to sample the difference signal S.sub.4 in gate circuits 43 and 44 and values thus samled are held in hold circuits 45 and 46, respectively. The sampled and held signals S.sub.7 and S.sub.8 have been shifted from the RF signal range to the tracking error signal range by means of the sampling and holding process. Moreover, the signals S.sub.7 and S.sub.8 have a bipolar signal relationship with respect to the tracking center. Therefore, the signals S.sub.7 and S.sub.8 contain the tracking error components. In order to effect an enhancement, a difference between the signals S.sub.7 and S.sub.8 is derived by a subtractor 47 to produce a final tracking error signal S.sub.9.
In order to obtain the focusing error signal, a sum of the outputs a and b and a sum of the outputs c and d are formed by adders 48 and 49, respectively, and then a difference [(a+b)-(c+d)] between these sums is determined by a subtractor 50. The determined difference [(a+b)-(c+d)] is passed through a low pass filter 51, thereby producing the focusing error signal.
The tracking error detection system shown in FIGS. 1 and 2 is referred to as a differential system and the tracking error signal derived by this differential system is represented by DFF, while the tracking error detection system illustrated in FIGS. 4 and 5 is referred to as a heterodyne system and the tracking error signal obtained by the heterodyne system is expressed by HTD.
In FIG. 6, a comparison between DFF of the differential detection system and HTD of the heterodyne detection system is made.
As explained above, when the objective lens or the reflection mirror is moved in accordance with the tracking error, with the light detector being fixed, or when the rotating record disc is tilted, the reflected or transmitted light beam incident upon the light detector is shifted. It is now assumed that an amount of this shift is .DELTA.r.
In the left column of the chart shown in FIG. 6, the RF signal, HTD and DFF in the case of .DELTA.r=0 is shown, and in the right column, the RF signal, HTD' and DFF' in case of .DELTA.r.noteq.0 is shown.
It is apparent from FIG. 6 that when the beam is shifted, HTD and DFF are changed by .DELTA.HTD and .DELTA.DFF, respectively. The .DELTA.HTD and .DELTA.DFF are error components caused by the shifting of the beam and in order to accurately effect the tracking control, it is desired to decrease these disturbance components to be as small as possible.
FIGS. 7, 8 and 9 are charts of an intensity distribution of a diffraction pattern at the objective lens surface representing variations of RF, HTD, DFF, .DELTA.HTD and .DELTA.DFF in accordance with a variation in a positional relation between the pit on the record medium and the focused light spot. The charts were made with the assumption that the incident beam to the objective lens is a plane wave, the diameter of the beam spot is that of a cross section at which the intensity is decreased to 1/e.sup.2 of the maximum intensity at a center, and the pit has a length equal to the spot diameter, a width equal to a third of the spot diameter and a depth of .lambda./5, being a wavelength of the light beam. This depth corresponds to .lambda./2.5 for the transmission type record medium.
A t' axis represents an axis passing through the center of spot S and is tangential to the array of pits P, and an r' axis represents an axis passing through the spot center and extending in a radial direction of the information track. FIG. 7 represents the case of no tracking error, and FIGS. 8 and 9 are charts which illustrate tracking errors in opposite directions. The similar intensity distribution can be obtained in a region which may be considered to be a far field of the track. In the charts, the diameter of the beam spot is set to 4.3 mm and the beam is shifted by .DELTA.r=.+-.0.42 mm on the light receiving region. The amounts of RF, HTD, DFF, .DELTA.HTD and .DELTA.DFF are plotted as signal levels with reference to the light incident upon the objective lens and having a unit intensity.
From the charts shown in FIGS. 7, 8 and 9, it is apparent that the influence of the beam shift .DELTA.r appears largely in the differential system rather than in the heterodyne system. It should be noted that the variation of .DELTA.HTD and .DELTA.DFF is substantially proportional to the shift amount .DELTA.r.
In the case of the pit depth of .lambda./4, the diffraction pattern in the far field region is symmetrical with respect to an origin, so that HTD varies just like the case of the pit depth not equal to .lambda./4, but .DELTA.HTD is always zero. Therefore, the heterodyne system is preferably applied to the record medium having the pit depth of .lambda./4.
It should be further noted that .DELTA.DFF reaches a minimum when the spot S is symmetrical with respect to the pit P at t'=0 and thus, DFF reaches a maximum, as illustrated in FIGS. 8 and 9.
Now, the differential system will be further considered. In this system, DFF'=DFF+.DELTA.DFF is detected as shown in FIG. 6 and then DFF' is integrated by means of the low pass filter 13, as illustrated in FIG. 1, to derive the tracking error signal. On the other hand, the disturbance component .DELTA.DFF becomes large when the light spot is moved from the symmetrical position t'=0 as shown in FIGS. 7 through 9. Therefore, by effecting the integration, a large disturbance component is introduced in the tracking error signal. For instance, when a pit array having long intervals T between successive pits P, as shown in FIG. 10A, is scanned by the assembly, the tracking error signal is affected by the .DELTA.r shift to a greater extent than in a case of scanning a pit array having shorter intervals T', as shown in FIG. 10B. The inventors have recognized the fact that since the influence of the disturbance is a minimum at t'=0, if the tracking error information is detected only at this point, or shortly before or after, it is possible to obtain a precise tracking error signal without being affected by the .DELTA.r shift of the light beam spot, with respect to the light detector.